Means and method of operating electron multipliers



Aug. 30, 1938. P. T.F ARN$WORTH ,1

MEANS AND METHOD OF OPERATING ELECTRON MULTIPLIERS Filed Aug. 18, 1936 :2 Sheets-Sheet 1 WWW/"T -INVENTOR,

PH/LO 7; FARNSWORTH. I

BY 08f ATTORNEYS.

Aug. 30, 1938,.

P. 'r. FARNSWORTH 2,128,580 MEANS AND METHOD OF OPERATING ELECTRON MULTIPLIERS I ruwm. 18, 1936 jzsneet s sneetz p v Pos/r/o/v of I U U ammo/v.

- Pos/rlolv OF I ELECTRON. TIME I J INVENTOR, PH/LQ 7f FAPNSWQPTH ATTORNEYS.

Patented Aug. 30, 1938 PATE NT OFFICE MEANS AND METHOD OF OPERATING ELEC- .TRON MULTIPLIERS Philo T. Farnsworth, San Francisco,'Calii'., as-

signor to Farnsworth Television Incorporated, San Francisco, Calif a corporation of California Application August 18, 1936, Serial No. 96,614

11 Claims.

My invention relates-to a means and method of operating an electron multiplier, and particularly that form of electron multiplier having as fundamental structure a secondarily emissive cathode surface and an accelerating anode spaced from and paralleling the anode surface.

Fundamentally my invention has to do with the form of electron multiplier I term a multipactor, in that amplification is obtained by repeated and cyclical electron impacts with a. cathode surface adapted to emit secondary electrons at a ratio greater than unity, Ordinarily impacts take place only once per cycle ofthe impressed or primary frequency. k

The present invention relates to a method of there aremultiple electron impacts during each -cycle of operation'of the-device, thus giving rise to what I shall term an electron frequency. As a consequence, this electron frequency being greaterthan the primary frequency, harmonics having large power may be obtained.

Among the objects of my invention, therefore,-

are: To provide a'method of operating a multipactor to give multiple secondary electron generating impacts per primary cycle; to provide a method-of obtaining harmonics containing alarge power output in an electronmultiplier; to provide a methodof generating harmonics; to pro- "vide a means and method of regulating the harmonic output ina multipactor; to provide a method of controlling the length of path and consequently the time of flight inan electron multiplier; to provide a method of operating a multipactor wherein there is a single secondary electron emissive surface; to provide a method of operating a cylindrical electron multiplier wherein the electron paths are substantially radial; to provide a method of confining the time of flight 40 to a short period compared to the primaryperlod of an electron multiplier; and to provide a simple and efficient method of operation of an electron multiplier, having a single anode and a single Q within the scope of the appendedclaims.

- operating the device with an electron transit time operating a multipactor in such a manner that Referring to the drawings Figure 1 is a sectional view of amultipactor structure which may be used to practice my method, together with a schematic circuit indicating the proper operative connections.

Figure 2 is a diagram of a. diode multipactor operating where the transit time is the'same as" the primary frequency.

Figure 3 is a graph showing the position of the electron with respect to time in a multipactor operated in the circuit of Figure 2.

Figure 4 is a diagram showing a diode multipactor together with an auxiliary electrode for short relative to the primary period.

Figure 5 is a graph showing the position of the electron with respect to time as shown by the operating characteristics of the oscillator shown in Figure 4. p

Figure 6 is a graph showing the relative characteristic curves of a multipactor connected as shown in Figure 4. 7 7 I Figurefl is the over-all power-voltage characteristic of the. multipactor oscillator.

This application is a continuation in part of my 5 application Serial No. 80,193 filed May 16, 1936, wherein an auxiliary electrode was described as utilized to collect ions in a space wherein electron multiplication was taking place. Some gas ions f are always present, and when-alkali metal sur- 30' faces are used for the creation of secondary electrons, metal ions will also be present within the envelope containing the multipactor structure. During the operation of the device, these ions tend to bombard thecathode, .thus tending to 35 .tion,'bu't in the" present application, while said 45 auxiliary electrode is used for ion collection, it is also positioned and energized to control the length of the electron paths within the multipactor and thus control its time of flight. It is" obvious that asan ion collector this auxiliary I electrode may be positioned and energized in other locations and to other potentials than are H herein to be described, whereas in the present application this ion collector must have a certain position with respect'to the other electrodesfin f order to perform the method I am herein disclosing.

For a simple multiplier structure capable of practicing my present method of controlling the electron flight, direct reference is made to Figure 1, which is substantially the same structure as referred to in my prior application mentioned above. An envelope i is provided at oneend with a re-entrant stem 2 through which pass auxiliary electrode supports 3, supporting a central and preferably axial cylindrical grid structure d. This stem also supports by means of a peripheral ban 5 a cathode 6 in the form of an unperforated cylinder having its inner surface sensitized to produce secondary emission at ratios greater than unity when being impacted by electrons traveling at a velocity of 20 volts or greater, although, for the purposes of my method such sensitivity is not required but is desirable. I may, however, make this cathode of'a nickel barium alloy such as has-been described in my patent application Serial No. 70,714, flied March 24,1936. g

The opposite end of the tube also has a reentrant stem I sealed thereto supporting a gridlike anode structure 8, which may be formed as shown of the spiral wire welded to supports, or may be a perforated sheet metal assembly, the main point being that it should. be perforated to allow electrons to pass toward the axis of the assembly. Anode 8 is energized to a high potential. preferably several thousand volts, by

means of anode source 9, the negative end being;

grounded. The auxiliary electrode is energized through an auxiliary source I0, preferably vari- ,able, this source energizing the auxiliary electrode to a negative potential, the positive end being grounded. The cathode 8 has a lead ii passing through the wall of the envelope, this lead going to a tuned circuit ii, the, other end being grounded, thus placing a tuned circuit directly across anode and cathode andproviding the main or primary frequency. An output circuit i4 is coupled to the tuned circuit l2, and this output circuit'may lead to transmission line or other device as will be explained later. The device is now ready for operation as a selfoscillator if desired, or may be driven if desired, and in this latter instance coil I4 is used to supply the primary frequency between the cathode and anode. i

To more fully understand the method of operation of my device I prefer torefer directly to a diagram as shown in Figure 2, which represents a fundamental electron multiplier structure using a single anode and a single cathode, 'wherein the time of flight of the electron determines the output frequency. In this'case the tuned circuit i2 is designed to have a period equal to a time of flight which is substantially diametrical, and in addition; anode source 91s adjusted so that the acceleration given to the electron will bring the electron across the cathode space in the period selected. In other words, the potential of anode source 9 and the period of tuned circuit l2 should be adjusted so that the. time of flight corresponds to the period. a

In this type of multiplier the anode 8 is made considerably smaller than the cathode 6, thus giving an acceleration toward the axis of the device. When hooked up in this .way the position of the electron with respect to time will be that. as shown in Figure 3, the electrons leaving the cathode at one point, passing near the 'axis of the cathode cylinder 6 and impacting the amaeso opposite side of the cathode to produce secondaries, these secondaries then returning past the axis to again impact the cathode on an opposite suriace, as indicated by dash lines it in Figure 2. This action, of course, takes place along all diameters and the output current will have its greatest power in the primary frequency as determined by the adjustments and by tuned circuit I2, and there will not be a large harmonic content.

. My present method of operation however, is quite different. It will be seen that in the operation of the simple multipactor just described immediately above, there is only one secondary electron generating impact per primary cycle, and I have therefore operated my device in an entirely difierent manner as shown in Figures 4, 5-, 6 and 7. Figure 4 will be found to be a diagram representing schematically a sectional view of Figure l and in this case I have shown the electron path i5 as being a radial path rather than a diametrical path. This is due to the fact that the anode 8 is placed relatively close to the emitting surface of cathode 6 and is therefore much larger in diameter, and that there is a negative potential on the inner auxiliary electrode 4. Thus, as an electron leaves the cathode it is accelerated by the anode and passes therethrough toward the axis and toward the negatively charged electrode 4, which rapidly decelerates the electron which, in addition, is being decelerated by traveling away from anode 8. The combination of these two potentials turns the electron around, drives it back through the anode to impact the cathode again at or'near the cathode surface at which it originated, and it is quite obvious that the length of the path I! can be controllediby amount of the negative potential impressed upon auxiliary electrode 4', and that if the length of the path is changed the time of flight will be changed without, how-,

ever, changing the primary frequency.

The reason for this is shown in Figures 5, 6 and '1. Figure 5 is a graph showing the position of the electron with respect to time in my new method of the operation of the device, showing I that there are multiple impacts with the cathode and that these impacts will be greater innumber than the primary frequency.

In Figure ,6 I have shown all three characteristic curves of the multipactor operating under dicated by dash line "E'? where secondary emission will cease. I have found that no difficulty is encountered in obtaining, for example, maximum secondary emission ratios of four-to-one at the highest production of secondaries. At the minimum difference of potential between anode and cathode (point E) the emission of secondaries ceases and the electrons generated oscillate back and forth through the anode, gradually lose velocity and are collected at the electron frequency, thus giving rise to an output current as shown in curve C, the rising portion of this 75 justing the potential on auxiliary electrode 4 4 the number of impacts per primary cycle can be readily controlled and thus the amount of current in any desired harmonic may be ob; tained. In that respect output circuit I 4 as shown in Figure 1 may be utilized to draw one or more harmonics as desired to be used for predetermined purposes.

So high in power are the harmonicsv developed by operating the device in this manner that I have been able to obtain five or ten per cent of the output power at the tenth or twentieth harmonic. The property of.harmonic generation is, of course, highly valuable for use in master oscillators or for ultra high-frequency work because the primary frequency may be relatively low' and positively controlled'much easier than can the harmonics themselves. Furthermore, the anode conversion efliciency of a multipactor operated as I have just described is extremely high, and for that reason the heat production is low, thus allowing high powered structures to be utilized in relatively small envelopes.

I claim: I i

1. In an electron multiplier having an envelope containing a cathode adapted to emit secondary .electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode adjacent said cathode. the method of operation which comprises impressing a primary alternating potential between anode and cathode of sufficient intensity to cause electrons from said cathode to pass through said anode, and returning said electrons through said anode a plurality of times during a single period of the primary frequency.

2. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode adjacent said cathode, the-method of op-, eration which comprises impressing a primary alternating potential between anode and cathode of sufilcient intensity to cause electrons from said cathode to pass through said anode, and returning said electrons through'said anode to \contact said cathode a plurality of times during a single period of .the primary frequency.

.3. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode adjacent said cathode, the method of operation which comprises impressing a primary alternating potential between anode and cathode of sufflcient intensity to cause electrons from said which comprises impressing a primary alternating potential between anode and cathode of suflicient intensity to cause electrons from said cathode to pass through said anode, and cyclically re-.

turningsaid electrons through said anode at a secondary frequency greater than the primary y frequency.

5. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode adjacent said cathode, the method of operation which comprises impressing a primary alternating potential between anode and cathode of sumcient intensity to cause electrons from said cathode to pass through said anode, cyclically returning said electrons through said anode at a secondary frequency greater than the primary frequency, and collecting a portion of the electrons on the anode during each passage therethrough.

6. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode quency, collecting a portion of the electrons on the anode during each passage therethrough, and utilizing the collected current at said secondary frequency. '7. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith, and a perforated anode adjacent said cathode, the method of operation which comprises impressing a primary alternating potential between anode and cathode of suflicient intensity to cause electrons from said cathode to pass through said anode, subjecting said electrons to a negative charge directed to return said electrons through the anode a plurality of times during a single period of the primary period, collecting a portion of the electrons on the anode at each passage therethrough to create a secondary fre-, quency, and utilizing said secondary frequency.

8. In an, electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at a ratio greater than unity upon electron impact therewith; and a perforated anode adjacent said cathode, the method of operation which comprises impressing a primary alternating potential between anode and cathode of sum-- cient intensity to cause electrons from said oath ode to pass through said anode, subjectingsaid electrons to a negative charge directed to return said electrons through the anode a plurality of times during a single period of the primary period,

collecting a portion of the electrons on the anode at each passage therethrough to create a second-. ary frequency, and varying the amount of said charge to vary said secondary frequency.

9. In an electron multiplier having an envelope containing a cathode adapted to emit secondary electrons at .a ratio. greater than unity upon elec-' tron impact therewith, and a perforated anode adjacent said cathode, the method of operation which comprises impressing a primary altemating potential between anode and cathode of sufficient intensity to cause electrons from said cathof generating secondary electrons at a ratio greater than unity which comprises moving electrons away from and'against said surface capable of emitting secondary electrons at a ratio greater than unity and at a predetermined frequency to create successive secondary producing impacts, changing the impact velocity of each successive impact in accordance with a second predeter- 20 mined frequency to create multiple impacts per v tioned frequency.

amaseo cycle of said first predetermined frequency, and collecting the generated electrons at the first men- 11. The method of electron multiplication utilizing electron impact with a surface capable of generating secondary electrons at a ratio greater than unity which comprises moving electrons away from and against said surface'capable of emitting secondary electrons at a ratio greater than unity and at a predetermined frequency to create successive secondary producing impacts.

changing the impact velocity of each successive impact in accordance with a second predetermined frequency to create multiple impacts per cycle of said first predetermined frequency, collecting the generated electrons at the first men-" tioned frequency, and in varying amounts in accordance with the last mentioned frequency.

PHILO v '1. FARNSWORTH. 

